A third generation cellular system uses radio network controllers (RNC) and base stations (BS). Traffic from the core network to the user (downlink) is routed by the RNC to the BS(s) best able to serve a given user. The data sent by the RNC to the BS for a given user is segregated into transport channels (DCHs), each having their own characteristics in terms of coding type, rate and interleaving. The BS periodically collects the data in the form of transport blocks from the RNC, applies the appropriate coding and interleaving for each DCH, multiplexes the data from these different DCHs, and transmits it on the appropriate physical channel (DPCH) or DPCHs. The DPCH(s) is defined in terms of spreading code and, in case of discontinuous transmission (TDD), timeslot.
The power at which signals are transmitted on a given DPCH for a given user depends on several factors such as the path loss between the user and the BS, the interference level perceived by the user, and the signal-to-interference ratio (SIR) required for satisfactory transmission. The required SIR for a given user in a given DPCH may depend on the amount of data to be transmitted from the different DCHs during a specific time period (frame). This amount of data can be referred to as a transport format combination (TFC).
The user periodically requests the BS to adjust its power up or down using an uplink channel as the SIR it experiences varies down or up, respectively. The BS may decide to satisfy the request from the user or not. In a given timeslot, the total power used to transmit all DPCHs cannot exceed a certain threshold. If the BS finds itself in a situation where it is about to violate the threshold, the BS has to reduce the power of every DPCH (by the same relative amount) to avoid exceeding the threshold. This situation is referred to as a power outage.
Congestion is a general term that encompasses any situation where the BS is not able to satisfactorily transmit all the data sent by the RNC to the various users or user equipment (UE) connected to the BS. This may be due to a lack of hardware resources, processing power or transmission power. In order to provide a mechanism for the BS to prioritize between DCHs in case of congestion, the RNC assigns a priority to every DCH: the frame handling priority (FHP). The FHP is assigned by the RNC and transmitted to the BS so that the most important data gets through in case of congestion.
This approach, however, fails to indicate how much low priority data has to be removed to ensure avoidance of severe power outage. A method and system are therefore needed for removing the optimal amount of data in case of congestion, taking into account the power requirements and the FHP of the data.